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View Article OnlineView Journal | View Issue

Spiked gold nano

aInstitute for Chemistry, University of Potsd

25, 14476 Potsdam, Germany. E-mail: koetzbChemistry Department, Faculty of Science,cInstitute for Physics, University of Potsdam

14476 Potsdam, GermanydSchool of Analytical Sciences Adlershof (S

Albert-Einstein-Str. 5-9, 10099 Berlin, GermeDepartment of Biomaterials, Max Planck

Muhlenberg 1, 14476 Potsdam, GermanyfInternational Scientic-Educational Cente

Baghramyan Ave. 24d, 0019 Yerevan, Armen

† Electronic supplementary informa10.1039/d0ra00729c

Cite this: RSC Adv., 2020, 10, 8152

Received 23rd January 2020Accepted 12th February 2020

DOI: 10.1039/d0ra00729c

rsc.li/rsc-advances

8152 | RSC Adv., 2020, 10, 8152–8160

triangles: formation,characterization and applications in surface-enhanced Raman spectroscopy and plasmon-enhanced catalysis†

Ferenc Liebig,a Radwan M. Sarhan,bcd Matias Bargheer, c Clemens N. Z. Schmitt,e

Armen H. Poghosyan,f Aram A. Shahinyanf and Joachim Koetz *a

We show the formation of metallic spikes on the surface of gold nanotriangles (AuNTs) by using the same

reduction process which has been used for the synthesis of gold nanostars. We confirm that silver nitrate

operates as a shape-directing agent in combination with ascorbic acid as the reducing agent and

investigate the mechanism by dissecting the contribution of each component, i.e., anionic surfactant

dioctyl sodium sulfosuccinate (AOT), ascorbic acid (AA), and AgNO3. Molecular dynamics (MD)

simulations show that AA attaches to the AOT bilayer of nanotriangles, and covers the surface of gold

clusters, which is of special relevance for the spike formation process at the AuNT surface. The surface

modification goes hand in hand with a change of the optical properties. The increased thickness of the

triangles and a sizeable fraction of silver atoms covering the spikes lead to a blue-shift of the intense

near infrared absorption of the AuNTs. The sponge-like spiky surface increases both the surface

enhanced Raman scattering (SERS) cross section of the particles and the photo-catalytic activity in

comparison with the unmodified triangles, which is exemplified by the plasmon-driven dimerization of

4-nitrothiophenol (4-NTP) to 4,40-dimercaptoazobenzene (DMAB).

Introduction

Gold nanoparticles have received a great deal of attention due totheir special optical properties.1,2 One special feature of noblemetals on the nanometer scale is the localized surface plasmonresonance (LSPR).3,4 The LSPR strongly depends on the shapeand size of the gold nanoparticles, e.g., nanospheres,5 nano-rods,6 nanotriangles,7,8 nanostars,9–12 or multibranched nano-particles,13 and opens new elds of application.14,15 Asymmetricparticles allow for creating very large absorption and scatteringcross sections in the near-infrared (NIR) region, which is ofspecial interest for surface enhanced Raman scattering (SERS)

am, Karl-Liebknecht-Strasse 24-25, Haus

@uni-potsdam.de; Tel: +49 331 977 5220

Cairo University, Cairo 12613, Egypt

, Karl-Liebknecht-Strasse 24-25, Haus 27,

ALSA), Humboldt-Universitat zu Berlin,

any

Institute of Colloids and Interfaces, Am

r of National Academy of Sciences, M.

ia

tion (ESI) available. See DOI:

applications in biological systems. Gold nanostars with theirspike tips are of special relevance in this eld.16–19 Liz-Marzanet al. have shown that a Raman active molecule, sandwichedbetween the spike tip and a plasmonic surface, can enhance theSERS signal up to two orders of magnitude.19 Not only thesharpness of the tips is of relevance, but also the arrangementof the spikes to each other which tunes the plasmon resonance.Sheen Mers et al. studied the effect of spike length on the SERSsignal enhancement18 and Atta et al.10 investigated the seedmediated growth of 6-branched gold nanostars with very thinspikes up to a length of 100 nm. The interplay of surfactant,ascorbic acid (AA), AgNO3 and seed concentrations are of highimportance for the crystallization of the spikes. Upon excitationwith light at the tip apexes of nanostars large electromagneticelds can be localized, which can be exploited by SERS.19

Nanorods and nanotriangles are another class of anisotropicnanoparticles which offer excellent properties in electro-catalysis and Raman scattering due to the large electromagneticelds at the ends of nanorods20 or vertices of nanotriangles.8,21

For gold nanotriangles (AuNTs) the sharpness of the edges andtips is of special relevance for improving the enhancementfactor, and therefore, the application in catalysis. AuNTs couldbe produced by using different strategies, e.g., by a silver-freegrowth of CTAC-coated seeds in the presence of iodideanions7 or a seedless synthesis through oxidative etching.22Note

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that different chemical compounds can be used as SERSsubstrates, e.g., graphene oxide/AgCo composite nanosheets asefficient catalysts for dimerizing p-nitrothiophenol.23 However,a novelty of gold-based SERS-active nanostructures is theirutilization in biomedical sensing.24 AuNTs with an absorptionmaximum at about 1200 nm, in the second window for in vivoimaging,25 are of special relevance in this eld of research.26

Taking this into account, an extraordinary enhancementeffect can be expected, when the spike and anisotropic strate-gies come together. First examples of such a concept wereshown by Liz-Marzan et al. by growing tips on gold nanorods.27

Due to the antenna effect from the spikes higher SERS signalswere obtained. Recently, it was shown by different groups thatislands produced in the seed-mediated overgrowth28 or bya partial surface passivation29 of nanotriangles lead to excellentproperties in surface-enhanced Raman scattering.

Here we present a new family of spiked gold nanotriangles(SAuNTs) based on ultra-at (7.5 nm) nanotriangles8,30 deco-rated with gold spikes. The idea is to crystallize gold nano-particles directly on the {111} gold surface of nanotrianglesunder similar conditions already used for making nanostars.While the reduction of Au with AA in the AOT (dioctyl sodiumsulfosuccinate) shell of the nanotriangles leads to an undulatedsurface modication, adding the directing agent AgNO3 trans-forms the undulations into sharp spikes growing on the plateletsurface. As a result, SAuNTs with a 75 times higher enhance-ment factor in SERS experiments become available.

Experimental sectionMaterials

Tetrachloroauric(III)acid (HAuCl4$3H2O), silver nitrate (AgNO3),dioctyl sodium sulfosuccinate (AOT) (98%) and 4-nitro-thiophenol (4-NTP) were purchased from Sigma Aldrich. L(+)-Ascorbic acid (AA) ($99%) were obtained from Roth. Milli-QReference A+ water was used in all experiments. The phospho-lipid PL90G with a purity >97.3% was obtained from Phospho-lipid GmbH.

Methods

A Shimadzu UV-2600 spectrophotometer was used in thewavelength range between 200 and 1400 nm to obtain UV-vis-NIR absorption spectra. The morphology of the different goldnanoparticles was identied by using a transmission electronmicroscope JEM-1011 (JEOL) at an acceleration voltage of 80 kVas well as the JEM-2200 FS (JEOL) at 200 kV for high resolution(HRTEM), fast Fourier transformation (FFT) and elementmapping (EDX measurements). For obtaining SEM micro-graphs a ZEISS Supra 55PV scanning electron microscope at anacceleration voltage of 2 kV was used. The zeta potential wasdetermined with a Malvern Nano Zetasizer 3600. The SERSperformance was assessed using a confocal Raman microscopeAlpha 300 (WITec) coupled with laser excitation at wavelengthof 785 nm. The 2 and 10 mW laser beam was focused on thesample through a Nikon 20� objective lens. The Raman spectrawere acquired with a thermoelectrically cooled Andor CCD

This journal is © The Royal Society of Chemistry 2020

detector DU401A-BV placed behind the spectrometer UHTS 300from WITec with a spectral resolution of 3 cm�1, and wererecorded with an integration time of 2 seconds, which is alsothe exposure time of the laser beam. The Raman band of thesilicon wafer at 520 cm�1 was used to calibrate thespectrometer.

Nanoparticle synthesis

Synthesis of AuNTs. The synthesis of the AuNTs was ach-ieved according to our recently published protocol.8 A vesiclephase containing 0.5 wt% phospholipid PL90G and 0.5 wt%AOT solution was produced and subsequently mixed witha 2 mM tetrachloroaurate precursor solution. Aer shaking10 mL of the vesicular template phase (containing 0.05 g PL90G;0.05 g AOT; 9.9 g water) for two days, 30 mL of the 2 mM goldchloride solution were added. Aer heating this mixture 45minutes at 45 �C, a polydisperse gold nanoparticle solution wasobtained. For the separation of the AuNTs from spherical ones,a depletion occulation was performed by adding a 0.02 M AOTsolution. Aer 3 washing and centrifugation steps (10 minutesat 4000 rpm) the resulting green colored AuNTs solution witha zeta potential of �59 mV was used for further experiments.The AuNTs solution concentration was determined by SAXS andis approximately 0.16 mgmL�1 with an estimated uncertainty of5%.

Synthesis of SAuNTs. The AuNTs solution was 4 timesdiluted to a used stock solution concentration of 0.04 mg mL�1.To 300 mL of the AuNTs stock solution we have added 900 mL ofa 2 mM gold chloride precursor solution and 45 mL of a 2.5 mMAgNO3 solution. For the reduction process 600 mL of a 0.1 M AAsolution was added under fast stirring to ensure a homogenousdistribution of the reduction agent leading to a monodispersespike formation. Because of the very fast reaction, the stirringtime was 30 seconds. The samples were characterized by UV-vis-NIR spectroscopy, SEM and TEM microscopy.

SAuNTs monolayer formation and surface functionalizationwith Raman active linker molecules. For SERS measurements,50 mL of the nal SAuNTs solution was dropped on a siliconwafer. The concentration of the nal SAuNTs solution was notdetermined, but aer the centrifugation steps and the remov-ement of the sphere- or star-like ones, the concentration shouldbe almost the same as the stock AuNTs solution. The addition of25 mL of the ethanol–toluene-mixture (volume ratio 5 : 1) to thedroplet leads to a lm casting process as described by usearlier.31 The evaporation process was done at room tempera-ture under a covered glass with air slits at the bottom. Thesilicon wafer was immersed in 10 mM 4-nitrothiophenol (4-NTP) ethanolic solution for 3 hours aer the solvent evapora-tion, and was washed several times in ethanol to remove thephysisorbed molecules.

Construction and MD simulation details

MD simulations of the adsorption of AA at the surface ofAOT-stabilized AuNTs. We build on the MD simulations theAOT bilayer composed of 84 molecules adsorbed on Au(111)facets.32,33 Aer addition of 50 AA molecules this system was

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optimized for 5000 steps. A 100 ns production run was carriedout and the last 50 ns of production trajectories were used fordata collection and further analysis. Note that the statisticindependent run (100 ns) was also done.

Two simulations were carried out using GROMACS sowarepackage and the CHARMM27 all-atom force eld protocols forthe AOT surfactant, described by Abel et al.34 Note that theGOIP-CHARMM all atom force eld was applied to describe thegold surface,35 already used by us previously.32,33 The forceeldfor AA molecules was retrieved with the CGenFF databaseserver,36 which generates the CHARMM force eld for smallorganic molecules. In two simulations, the SPC model37 wasused for water molecules. The room temperature was main-tained by applying a Nose–Hoover thermostat,38 and thetemperatures for all components were controlled indepen-dently. The LINCS approach39 was applied to x all bonds. ThePME method40 (truncation at 1.2 nm) was used for electrostaticinteractions and the van-der-Waals interactions were cut at1.2 nm, too. The equation of motion was integrated using theVerlet-leapfrog approach41 with a timestep of 1 fs.

The clusters available at IIAP NAS RA42 were used. Additionalcomputational resources were obtained from http://bioinformatics.am. All visualizations in the text have beenrepresented by VMD graphical package.43

MD simulations of the adsorption of AA at the surface ofa gold cluster. A gold sphere consisting of 144 Au atoms44 wassolvated in bulk water in the presence of 50 AA molecules.Further steps and simulation details were the same as describedabove. The production run for 50 ns was carried out.

Results and discussionFormation of SAuNTs

The general concept for the synthesis of SAuNTs is shown inScheme 1, where AuNTs have been decorated with spikes inpresence of AgNO3, HAuCl4 and AA.

AuNTs synthesis

For the synthesis of AuNTs, mixed AOT/phospholipid vesiclesact as a template phase.8 The yield of produced gold nano-platelets is 33% with a polydispersity of 33.3%.8 Because of thepolydispersity, a purication step was necessary to separate thenanoplatelets from spherical particles. This was done by anincrease of the AOT concentration above the critical

Scheme 1 Approach to SAuNTs.

8154 | RSC Adv., 2020, 10, 8152–8160

micellization concentration (cmc) to perform a depletion oc-culation leading to a sedimentation and isolation of the nano-platelet fraction.8 Aer this purication step EDXmeasurements reveal (by the presence of a sulfur-peak) thatAOT molecules are located on the platelet surface.45 Accompa-nying molecular dynamics (MD) simulations propose theadsorption of AOT micelles and bilayers on the {111} goldsurface.32,33 The excess of AOT molecules in the solution couldbe removed by centrifugation. 79% of the nanoplatelets(including hexagonal and triangular nanoprisms) are long-termstable AOT-coated AuNTs with an average edge length of 175 �17 nm.8 A special feature of the ultrathin nanoplatelets witha thickness of 7.5 � 1 nm, calculated by different methods, i.e.,X-ray reectivity, TEM tomography,46 SAXS measurements,30

and veried in ultrafast X-ray diffraction experiments,47 is theirlong-term-stability up to a laser uence of 2.9 mJ cm�2,47 andtheir ability to form monolayers.8,31

In the following we call these ultrathin nanoplatelets AuNTs,because of the fact that 79% of the platelets are triangularly-shaped. Note, that the term nanotriangle is oen used fortriangular nanoprisms, too.48

Due to the existence of a more rigid adsorbed AOT bilayerwith a lower rate of AOT diffusion (�10�10 cm2 s�1 calculatedfrom MD simulations),33 which act as stabilizer, the zetapotential is �59 mV. The absorption spectrum (Fig. 1) showsa maximum at 1280 nm in the NIR-region, which is essential forin vivo imaging of biological tissues.25 A TEMmicrograph of theAuNTs is shown in Fig. 1 and a low magnication SEM image inFig. S1.†

AuNTs surface modication in the absence of AgNO3.

Rough surfaces may improve the SERS enhancement factor bythe lightning-rod effect and the photocatalytic activity viahigher curvature of the surface. Therefore, we have tried todecorate the AuNTs surface with gold nanoparticles by reducinga gold chloride solution with AA. TEM micrographs of theAuNTs aer treatment with the gold chloride solution in pres-ence of AA show the formation of spherical gold nanoparticlesat the surface of the triangles resulting in an undulated surfacemodication (Fig. 1, green marked TEMmicrograph) analogouswith our previous experiments by making gold half-spheres ina PEI shell of AuNTs.49 This process is accompanied by a shi ofthe UV absorption from 1320 nm of the initial bare AuNTsdispersion to the broad maximum of around 1150 nm, which

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Fig. 1 UV-vis-NIR absorption spectra with corresponding TEM micrographs of bare AuNTs (red color) in comparison to the spectra and TEMmicrographs after modification in absence of AgNO3 (green color) and in presence of AgNO3 (blue color).

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can be assigned to an increase of platelet thickness. In contrastto the bare AuNTs system one can nd two further peaks in theUV-vis spectrum at 580 nm and 680 nm. TEMmicrographs showin addition to the undulated AuNTs signicant smaller cube-like structures, which can be responsible for the UV absorp-tion at about 600 nm. To verify this assumption, we have per-formed additional experiments in absence of AuNTs.

By reducing HAuCl4 directly with ascorbic acid in absence ofAuNTs cube-like nanoparticles with small rounded spikes areformed. The UV-vis spectrum of the dispersion shows two peaksat 580 nm and at 720 nm. The rst peak can be assigned to theformation of spherical particles with diameter of around110 nm and the second one at about 720 nm can be related tostar-like particles, as to be shown by the corresponding TEMmicrograph (Fig. S2†).

Therefore, we can conclude that the absorption in the regionbetween 500 and 800 nm in Fig. 1 (green curve) can be related tothe formation of separately formed gold nanoparticles in solution.

SAuNT formation in presence of AgNO3

Based on previous experiments,50 we know that in an aqueousmicellar AOT solution in the presence of AgNO3 as a shape-

This journal is © The Royal Society of Chemistry 2020

directing agent and AA as reduction agent gold nanostars canbe synthesized by the reduction of HAuCl4. Designed spikes canbe synthesized in a micellar AOT solution by adding 1.5 mL ofa 2 mM HAuCL4 and 80 mL of a 2 mM AgNO3 solution.50 In thiscase, it's possible to achieve particles of around 75 nm withsharpened spikes and an absorption maximum at 660 nm(compare Fig. S3†). Therefore, the following experiments wereperformed at similar conditions, optimized for the spikeformation process.

It is well established that silver salts inuence the goldnanoparticle formation. For example, the second step in theseed-mediated synthesis of nanorods is performed in presenceof AgNO3.51 Recently, it was shown by Atta et al. that the silvernitrate concentration is of high importance for making 6-branched gold nanostars in presence of the nonionic surfactantTriton X.10

In presence of AgNO3 the AuNTs are covered by spikes, whichare homogenously distributed over the whole surface area of theAu nanoplatelets (compare blue framed TEM micrograph ofFig. 1). Moreover, a small blue-shi of the absorption maximumto 1220 nm can be observed, which is related to the increasedplatelet thickness. The disappearance of an absorption peak

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maximum between 500 and 800 nm demonstrates that the goldnanoparticles are predominantly formed at the surface of theAuNTs and not in solution. This result can be understood by anelectrostatic attraction between the negatively charged AOTbilayer of the triangles and the positively charged Au3+ and Ag+

ions. In consequence the metal ions are attached to the AOTbilayer surrounding the AuNTs, and the gold reduction processis performed in the AOT bilayer.

To verify the morphology of the SAuNTs, HRTEM investiga-tions were made in combination with EDX analysis. The redarea of higher magnication in Fig. 2 shows that individualspikes are formed at the platelet periphery of the nanotriangles.The associated fast Fourier transformation (FFT) in Fig. 2 (bluemarked area) elucidates the crystalline structure of an indi-vidual tip. The pattern reveals a growth in [011] direction, whichcould also be determined for the spike growth of gold nanostarsin DMF with PVP as the symmetry breaking component.11 Thespots correspond to {200} and {111} reections. Therefore, itcan be concluded that the growth is preferred at the (100) and(111) facets leading to the formation of spikes in agreementwith results obtained for nanostars.

The EDX analysis of SAuNTs (Fig. 2, right side) allows us tocharacterize the surface coating with the relevant metals, i.e., Au(red colored) and Ag (green colored). Both metals cover thewhole surface. The intensity for silver is much less pronouncedand we estimate a 20% silver content in the spikes since the Ausignal in Fig. 2 partially originates from the bare AuNTs withoutspikes, which amounts to approximately half of the volume ofthe SAuNTs. This is in good agreement with EDXmeasurementsfrom Atta et al., where the authors have found metallic silver atboth side walls of the spike,10 as well our own EDX-line scanacross a gold nanostar-spike, synthesized in concentratedaqueous AOT solution.47

The absence of the absorption peak around 650 � 150 nm inthe synthesis of SAuNTs in presence of AgNO3 is related to thefact that the whole amount of Au3+ ions is required for the

Fig. 2 TEMmicrograph of a spiked gold nanotriangle at low (scale bar: 5Fourier transformation (FFT) of the marked area. EDX measurements oftuations in the intensity originating from the roughness of the platelets s

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formation of the spikes, meaning that no more precursor salt isavailable for making gold nanoparticles in solution.

In summary, AgNO3 affects the growth of the particlestowards sharper spikes, which are coated by metallic silver,whereas AOT affects the growth on the AuNT surface.

For a more comprehensive discussion of the platelet stabilitythe zeta potentials of the different systems are compared inTable 1. The samples with AgNO3 have a considerable highernegative zeta potential indicating a better electrostatic stabili-zation. The fact that silver could be found on the SAuNT surfacesuggests that silver builds a layer around the gold spikes.Although EDX measurements are not able to distinguishbetween Ag+ ions or Ag0, the increase of the negative zetapotential cannot be explained by the adsorption of positivelycharged Ag+ ions. Therefore, we assume that Ag+ ions arereduced on the spike surface via a underpotential deposition,and the corresponding NO3

� ions increase the negative zetapotential at the particle surface, which is in agreement with theresults shown by Atta et al.10 and Tebbe et al.6

Moreover, the use of AOT leads to a higher negative zetapotential of the AuNTs. In case of surface modication, thenegative zeta potential decreases. This could be explained by theformation of gold nanoparticles in the AOT bilayer shellattached to the AuNT surface. In consequence AOT moleculesare dismantled, which is accompanied by a decrease of thenegative zeta potential. Nevertheless, the absolute values of thezeta potential > 30 mV are indicative of an electrostaticallystabilized colloidal system. The increase of the roughness of theplatelet surface leads to an additional entropic repulsion effectdue to a decrease of the conformational arrangements betweenthe particles (non-DLVO interactions).52

MD simulations

Recently, we have shown by MD simulations that already aera few nanoseconds AOT bilayers adsorb on AuNT-surfaces.33 Byadding the reducing agent AA to the system gold clusters are

0 nm) and high resolution (scale bar: 5 nm) with the corresponding fastAu (red) and Ag (green) on the SAuNTs surface (right side) show fluc-urface due to the multitude of metal tips.

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Table 1 Zeta potential measurements of the different systems withand without AuNTs

SystemZeta potential[mV]

HAuCl4 + AA �15HAuCl4 + AgNO3 + AA �36HAuCl4 + AOT + AA �35HAuCl4 + AOT + AgNO3 + AA �42AuNTs �59AuNTs + HAuCl4 + AA �32AuNTs + HAuCl4 + AgNO3 + AA �39

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formed in the vicinity of the Au(111) facets, as to be shownearlier by HRTEM micrographs in the direct surrounding ofgold spikes from gold nanostars.50

Therefore, we have performed MD simulations with a goldcluster, put into the box with water in presence of 50 AA mole-cules. The gold cluster is already covered with AA moleculesaer some MD steps, where the oxygen atoms of AA, i.e., espe-cially the 3,4-dihydroxy groups at the hydrofuran-2-on ring,come close to the gold surface (compare snapshot in Fig. S4†).This result underlines why AA is a preferred reducing compo-nent in a seed mediated synthesis of anisotropic goldnanoparticles.

For a more comprehensive mechanistic discussion we havenow performed MD simulations by adding AA to the AOT-stabilized AuNTs. Before discussing the structural parameters,we visually inspected the layer-structure resulting in the simu-lations. Fig. 3 shows the snapshot extracted from the last frameaer 100 ns. While the AOT layer close to the Au surface ishomogeneous and at, well-pronounced vertical deviations ofAOT molecules appear in the upper layer.

Fig. 3 Snapshot from the last frame of MD simulation. The corre-sponding colors are: yellow – AOT, pink – gold surface. The AOTsulfurs and gold atoms were rendered as spheres, the water, coun-terions and hydrogen atoms were omitted for clarity, imitated via VMDpackage.43

This journal is © The Royal Society of Chemistry 2020

To explore the roughness, the vertical deviations of sulfuratoms from the layer's center of mass were checked. Note thatthe parameters were calculated as described in ref. 33, whichrepresents a vertical displacement (or average deviations) ofAOT sulfur atoms from layer center of mass.

For the lower layer close to the gold we track almost nochanges, while for the upper layer we see that the presence of AAmolecules leads to a signicant roughness of the layer. For thelower layer, the roughness value is almost the same �0.03 �0.005 nm, while for the second layer, in average, the roughnessvalue is twice higher (�0.23 � 0.01 nm) than without AA (0.13�0.01 nm), as compared with previous experiments.33 The AAmolecules attached directly at the AOT headgroups of thebilayer induce large uctuations.

Due to the bumpy surface and orientation of AA moleculesnear the AOT headgroup, the bilayer thickness or calculatedsulfur-to-sulfur distance is also changed from 1.76� 0.03 nm to1.60 � 0.01 nm. As a guide to interpret the numbers we plot theradial distribution function (RDF) of AA molecules (as well asAOT sulfur atoms) with respect to the gold surface in Fig. 4.

Note that the RDFs were calculated via the GROMACS g_rdfmodule from the last 1000 frames (last 50 ns) of both simulationand subsequently averaged over two simulations.

For AOT sulfur atoms of an AOT bilayer, it is obvious that therst layer is well-pronounced with adsorbed sulfur atoms closeto gold surface as seen previously,33 and the second one is muchmore diffuse and bumpy. For AA molecules, we nd one peak atthe distance of �2.4–2.5 nm from gold surface, i.e., one canconclude that the AA molecules are oriented near the uctu-ating AOT head groups.

SAuNT formation mechanism

Based on the results outlined here the mechanism of spikeformation onto the AuNTs can be discussed as follows:

In a rst step the positively charged Au3+ and Ag+ ions attachto the negatively charged AOT bilayer of the AuNTs by electro-static attraction. In a second step the added AA molecules areoriented near the uctuating AOT head groups. Therefore, thereduction process starts directly in the AOT bilayer due topreferred nucleation processes at Au(111) facets of the AuNTs.53

The following growth of spikes in [011] direction in the

Fig. 4 The radial distribution function (RDF) of gold surface to AAmolecules and AOT sulfur atoms.

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surrounding micellar AOT template phase is mainly related tothe blocking of certain facets by the silver ions. Ag+ ions arerequired for the spike formation in analogy to the silver-assistedgrowth of gold nanorods,54 and the underpotential depositionof silver ions on certain gold crystal facets.55

SERS performance of SAuNTs

A monolayer of SAuNTs was deposited on a silicon wafer bya recently described lm casting procedure.45 Fig. 5 shows theSEMmicrograph of the SAuNT-layer on the surface of the wafer.The monolayer without stacked particles is loosely packed incomparison to the “perfect” template monolayer of bare AuNTs(compare Fig. S1†), already shown by us earlier.45 Note that thenumber of regular nanotriangles is decreased from 79% to 65%during the spike formation process, which can be related toa partly deformation to non-dened structures. Therefore, thelower packing density can be related to an increased edgeroughness of the SAuNTs, which leads to a repulsion betweenthe individual nanoplatelets. Similar repulsion effects are wellknown from non-DLVO interactions between amphiphilicsurfaces due to an entropy effect related to the surfaceroughness.52

Metal nanoparticles composed of the coinage metals (mainlygold and silver), have been used in numerous applications suchas catalysis, nanomedicine, and photonics, owing to theiroptical properties that can be nely tuned by their localizedsurface plasmons.56,57 These collective oscillations of the parti-cles' electron density can enhance the electric eld, and there-fore, the optical signals of the molecules near the particle'ssurfaces, e.g. via SERS. Moreover, the decaying surface plas-mons produce highly energetic charge carriers (hot electron/hole pairs) as well as a local heat conned to the particlesthat are both known to initiate and/or enhance chemicaltransformations of the adsorbed molecules.58,59 These chemicaltransformations are termed to be plasmonically driven if theexcitation wavelength lies within the plasmonic absorptionband of the particles.60

Fig. 5 SEM micrograph of the SAuNT layer on the wafer surface (left sRaman spectra measured after adsorption of 4-NTP molecules on theintensity of the black spectrum is scaled by a factor of 10 to make it bet

8158 | RSC Adv., 2020, 10, 8152–8160

Here we use SERS to show the plasmon-induced catalyticperformance of our AuNTs. The dimerization of 4-NTP to DMABserves as a model system of the plasmonic reaction, while theSAuNTs are compared to the AuNTs regarding the photo-catalytic reactivity and the SERS enhancement. To this end,the 4-NTP molecules were self-assembled on the nanotriangles(bare AuNTs and SAuNTs) deposited on the silicon wafers. TheRaman spectrum (Fig. 5, right side) is dominated by the mainthree peaks of the 4-NTP at 1077, 1335, and 1575 cm�1, whichare assigned to the C–H bending, NO2, and the C]C stretchingmodes, respectively.60 Fig. S5† shows the main Raman signatureof the neat 4-NTP.

Fig. 5 shows the SERS spectra of 4-NTP molecules adsorbedon the surface of the AuNTs via the Au–S bond. The upper panelreports results measured at low laser power of 2 mW, with a verylarge enhancement of the Raman signature of 4-NTP moleculesadsorbed on the SAuNTs (red spectrum) in comparison to thoseadsorbed on the bare AuNTs (black spectrum). The peak area at1335 cm�1 obtained from the SAuNTs is approximately �20times larger than the one obtained from the bare AuNTs,despite the loss of the signal intensity because a considerablefraction of the reactant has been transformed to DMAB mole-cules, which show up at 1134 cm�1 for the N]N stretchingvibration. Consistently, the C–H bending/C–S stretching peakarea at 1077 cm�1, which is less affected by the reaction, showsa higher enhancement by a factor of �75.

A direct comparison of the absolute cross-sections with therelevant literature values is difficult, because of various types ofdye molecules under different reaction conditions (substrates ordispersions). For example, Scarabelli et al. have found anenhancement factor of 1.2 � 105 by using the nonresonantbenzenethiol in solution,7 and Kuttner et al. determined an EF formercaptobenzoic acid up to 5.6 � 104 at the half Au concentra-tion.48 Note, that the EF in solution even compares with EF ofdensely packed bare NT monolayers on solid substrates.

For the bare nanotriangles, an enhancement factor of EFAuNT ¼2.8 � 104 was found by assuming a monolayer of 4-NTP to be

ide) with the corresponding Raman spectra (right side). Upper panel:bare (black) and modified (red) AuNTs at a laser power of 2 mW. Theter visible. Lower panel: same for a laser power of 10 mW.

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adsorbed on the gold surface via the thiol bond.31 It is much moredifficult to estimate the surface of the SAuNTs in order to determinethe enhancement factor. As a very rough estimate, we approximatethe spikes as cones with an average height of 10 nm and a basediameter of 4 nm. This increases the surface by approximately 3,and yields an enhancement factor of about 8� 105 for the SAuNTs.

Next, we discuss the high plasmon-induced catalytic activity ofthe SAuNTs, which is evident from the appearance of strongRaman peaks at 1134, 1387, and 1434 cm�1 (marked with dashedlines in the red spectrum and listed in Table 2). These peaks,assigned to the C–N and N]N stretching modes of DMABmolecules, show the partial dimerization of 4-NTP into DMAB onthe surface of the SAuNTs.61 In contrast, the reaction product ishardly visible for the bare AuNTs (black lines of Fig. 5).

In order to quantify the increased reactivity of the SAuNT, weincreased the laser power to 10 mW. The lower panel of Fig. 5shows the SERS signal from SAuNT as a red spectrum. TheRaman peaks of the product DMAB at 1134 and 1434 cm�1 evensurpass the strongest peak of the reactant 4-NTP, which is theN–O stretching mode at 1335 cm�1. This conrms a very highreactivity of the SAuNT. We canmake a relative assessment of theincreased reactivity by noting that intensity ratio of the productand reactant peaks at 10 mW for the SAuNT is doubled withrespect to the ratio for the AuNT. For the lower uence of 2 mW,however, the reaction rate measured by this ratio is almostenhanced by two orders of magnitude. We tentatively attributethis large enhancement of plasmonic reactivity to the surfaceroughness at which the two reacting 4-NTP molecules can bearranged on the metallic surface. A quantication of the contri-bution of silver atoms on the surface to the reactivity andpotential lightning-rod effects require additional very chal-lenging systematic experimental investigations. Overall, weshowed that our SAuNTs are a promising candidate as a plas-monic catalyst and platform for SERS.

A direct comparison between bare AuNts and SAuNTs indi-cates the extraordinary effect of spikes with sharp tips on SERSperformance. Recently, we have shown that undulated AuNTswith gold half-spheres on the platelet surface show an inter-mediate enhancement effect, which is between bare AuNTs andSAuNTs. Therefore, we can conclude, that indeed the presenceof spike tips on the surface of NTs is of special relevance forRaman scattering. In that case Raman active molecules, i.e., 4-NTP, are sandwiched between the spike tip and a plasmonicsurface, as already proposed by Rodriguez-Lorenzo et al.19

Table 2 Raman wave numbers in relation to SERS assignments of 4-NTP and DMAB

Raman wave number(cm�1)

SERS assignmentsof 4-NTP

SERS assignmentsof DMAB

1077 C–H bendC–S stretch

1134 C–N stretch1335 NO2 stretch1387, 1434 N]N stretch1575 C]C stretch

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Conclusions

In this publication, the spike formation on the surface of ultra-at AuNTs is in the focus of research. The spike formation iscontrolled by the amount of AA and AgNO3. MD simulationsshow that the reducing agent, i.e., AA, can directly attach to theAOT-bilayer and to gold clusters, which is of special relevance forthe spike formation process in the AOT bilayer of the AuNTs, andnot in solution. The nal SAuNTs are covered bymetallic silver onthe spike surface, experimentally shown by EDX measurements.When comparing SAuNTs to bare AuNTs, we nd a relative SERSsignal enhancement of 75 and an increase of the plasmonicreactivity by a similar factor at low uences, whereas at highuences the catalytic activity is enhanced only by a factor of 2. Webelieve that this combination of the very intense NIR plasmonabsorption band of the AuNT with an edge length of about175 nm combined with the enhanced reactivity provided by thespikes, which are one order of magnitude smaller, are a superbplatform for improving plasmonic catalysis.

Conflicts of interest

The authors declare no competing nancial interest.

Acknowledgements

The nancial support of the German Research Foundation(INST 336/64-1) and the Open Access Publishing Fund ofUniversity of Potsdam are gratefully acknowledged.

References

1 M. C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.2 R. Sadar, A. M. Funston, P. Mulvaney and R. W. Murray,Langmuir, 2009, 25, 13840–13851.

3 K. L. Kelly, E. Coronada, L. L. Zhao and G. C. Schatz, J. Phys.Chem. B, 2003, 107, 668–677.

4 S. Eustin and M. A. El-Sayed, Chem. Soc. Rev., 2006, 35, 209–217.

5 S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, 4212–4217.

6 M. Tebbe, C. Kuttner, M. Mayer, M. Maennel, N. Pazos-Perez,T. A. F. Konig and A. Fery, J. Phys. Chem. C, 2015, 119, 9513–9523.

7 L. Scarabelli, M. Coronado-Puchau, J. J. Giner-Casares,J. Langer and L. M. Liz-Marzan, ACS Nano, 2014, 8, 5833–5842.

8 F. Liebig, R. M. Sarhan, C. Prietzel, A. Reinecke and J. Koetz,RSC Adv., 2016, 6, 33561–33568.

9 A. Swarnapali, D. S. Indrasekara, R. Thomas and L. Fabris,Phys. Chem. Chem. Phys., 2015, 17, 21133–21142.

10 S. Atta, M. Beetz and L. Fabris, Nanoscale, 2019, 11, 2946–2958.

11 P. S. Kumar, I. Pastoriza-Santos, B. Rodriguez-Gonzales,F. J. Garcia de Abajo and L. M. Liz-Marzan,Nanotechnology, 2008, 19, 015606.

RSC Adv., 2020, 10, 8152–8160 | 8159

RSC Advances Paper

Ope

n A

cces

s A

rtic

le. P

ublis

hed

on 2

5 Fe

brua

ry 2

020.

Dow

nloa

ded

on 2

/6/2

022

2:09

:40

AM

. T

his

artic

le is

lice

nsed

und

er a

Cre

ativ

e C

omm

ons

Attr

ibut

ion

3.0

Unp

orte

d L

icen

ce.

View Article Online

12 L. Shao, A. S. Susha, L. Shan Cheung, T. K. Sau, A. L. Rogachand J. Wang, Langmuir, 2012, 28, 8979–8984.

13 G. Maiorano, L. Rizello, M. A. Malvini, S. S. Shankar,L. Martiradonna, A. Falqui, R. Cingolani and P. P. Pompa,Nanoscale, 2011, 3, 2227–2232.

14 J. Zhao, X. Zhang, C. R. Yonzon, A. J. Haes and R. P. VanDuyne, Nanomedicine, 2006, 1, 219–228.

15 T. Endo, D. Ikeda, Y. Kawakami, Y. Yanagida andT. Hatsuzawa, Anal. Chim. Acta, 2010, 661, 200–205.

16 C. G. Khoury and T. Vo-Dinh, J. Phys. Chem. C, 2008, 112,18849–18859.

17 H. Yuan, C. G. Khoury, H. Hwang, C. M. Wilson, G. A. Grantand T. Vo-Dinh, Nanotechnology, 2012, 23, 075102.

18 S. V. Sheen Mers, S. Umadevi and V. Ganesh,ChemPhysChem, 2017, 18, 1358–1369.

19 L. Rodriguez-Lorenzo, R. A. Alvarez-Puebla, I. Pastoriza-Santos,S. Mazzucco, O. Stephan, M. Kociak, F. J. Gracia-de Abajo andL. M. Liz-Marzan, J. Am. Chem. Soc., 2009, 131, 4616–4618.

20 G. W. Bryant, F. J. Gracia-de Abajo and J. Aizpurua, NanoLett., 2008, 8, 631–636.

21 J. Nelaya, M. Kociak, O. Stephan, F. J. Gracia-de Abajo,L. Henrard, D. Taverna, I. Pastoriza-Santos, L. M. Liz-Marzan and C. Colliex, Nat. Phys., 2007, 3, 348–353.

22 L. Chen, F. Ji, L. He, Y. Mi, B. Sun, X. Zhang and Q. Zhang,Nano Lett., 2014, 14, 7201–7206.

23 X. Ma, Y. Guo, J. Jin, B. Zhao and W. Song, RSC Adv., 2017, 7,41962–41969.

24 T. Vo-Dinh, IEEE J. Sel. Top. Quantum Electron., 2008, 14,198–205.

25 A. M. Smith, M. C. Mancini and S. Nie, Nat. Nanotechnol.,2009, 4, 710–711.

26 F. Liebig, S. Moreno, A. F. Thunemann, A. Temme,D. Appelhans and J. Koetz,Colloids Surf., B, 2018, 167, 560–567.

27 P. Aldenueva-Potel, E. Carbo-Argibay, N. Pazos-Perez,S. Barbosa, I. Pastoriza-Santos, R. A. Alvarez-Puebla andL. M. Liz-Marzan, ChemPhysChem, 2012, 13, 2561–2565.

28 G. Wang, Y. Liu, C. Gao, L. Guo, M. Chi, K. Ijiro, M. Maedaand Y. Yin, Chem, 2017, 3, 678–690.

29 Q. Fan, K. Liu, J. Feng, F. Wang, Z. Liu, M. Liu, Y. Yin andC. Gao, Adv. Funct. Mater., 2018, 28, 1803199.

30 F. Liebig, A. F. Thunemann and J. Koetz, Langmuir, 2016, 32,10928–10935.

31 F. Liebig, R. M. Sarhan, M. Sander, W. Koopman, R. Schutz,M. Bargheer and J. Koetz, ACS Appl. Mater. Interfaces, 2017, 9,20247–20253.

32 A. Poghosyan, A. A. Shahinyan and J. Koetz, Colloids Surf., A,2018, 546, 20–27.

33 A. Poghosyan, M. P. Adamyan, A. A. Shahinyan and J. Koetz,J. Phys. Chem. B, 2019, 123, 948–953.

34 S. Abel, F. Stepone, S. Bandyopadhyay andM.Marchi, J. Phys.Chem. B, 2004, 108, 19458–19466.

35 L. B. Wright, P. M. Rodger, S. Corni and T. R. Walsh, J. Chem.Theory Comput., 2013, 9, 1616–1630.

36 K. Vanommeslaeghe, E. Hatcher, C. Acharya, S. Kundu,S. Zhong, J. Shim, E. Darian, O. Guvench, P. Lopes,I. Vorobyov and A. D. MacKerell Jr, J. Comput. Chem., 2010,31, 671–690.

8160 | RSC Adv., 2020, 10, 8152–8160

37 H. J. C. Berendsen, J. P. M. Postma, W. F. van Gunsteren andJ. Hermans, Intermolecular Forces, Reidel, Dordrecht, 1981.

38 S. Nose, J. Chem. Phys., 1984, 81(1), 511–519.39 B. Hess, H. Bekker, H. J. C. Berendsen and J. Fraaije, J.

Comput. Chem., 1987, 18, 1463–1472.40 T. Darden, D. York and L. Pedersen, J. Chem. Phys., 1993, 98,

10089.41 L. Verlet, Phys. Rev., 1967, 159, 98–103.42 H. Astsatryan, V. Sahakyan, Y. Shoukourian, P.-H. Cros,

M. Dayde, J. Dongarra and P. Oster, IEEE Proceedings of14th RoEduNet International Conference – Networking inEducation and Research (NER'2015), Craiova, Romania,September 24–26, 2015, pp. 28–33.

43 W. Humphrey, A. Dalke and K. Schulten, J. Mol. Graphics,1996, 14, 33–38.

44 E. Heikkila, A. A. Gurtovenko, H. Martinez-Seara,H. Hakkinen, I. Vattulainen and J. Akola, J. Phys. Chem. C,2012, 116(17), 9805–9815.

45 F. Liebig, R. M. Sarhan, C. Prietzel, C. N. Z. Schmitt,M. Bargheer and J. Koetz, ACS Appl. Nano Mater., 2018, 1,1995–2003.

46 N. Schulze and J. Koetz, J. Dispersion Sci. Technol., 2017, 8,1073–1078.

47 A. von Reppert, R. M. Sarhan, F. Stete, J. Pudell, N. Del Fatti,A. Crut, J. Koetz, F. Liebig, C. Prietzel and M. Bargheer, J.Phys. Chem. C, 2016, 120, 28894–28899.

48 C. Kuttner, M. Mayer, M. Dulle, A. Moscoso, J. M. Lopez-Romero, S. Forster, A. Fery, J. Perez-Juste and R. Contreras-Caceres, ACS Appl. Nano Mater., 2018, 10, 11152–11163.

49 F. Liebig, R. M. Sarhan, C. Prietzel, A. F. Thunemann,M. Bargheer and J. Koetz, Langmuir, 2018, 34, 4584–4594.

50 F. Liebig, R. Henning, R. M. Sarhan, C. Prietzel, C. N. Z. Schmitt,M. Bargheer and J. Koetz, RSC Adv., 2019, 9, 23633.

51 D. K. Smith and B. A. Korgel, Langmuir, 2008, 24, 644–649.52 D. Grasso, K. Subramaniam, M. Butkus, K. Strevett and

J. Bergendahl, Rev. Environ. Sci. Bio/Technol., 2002, 1, 17–38.53 G. Bogels, H. Meekes, P. Bennema and D. Bollen, J. Cryst.

Growth, 1998, 191, 446–454.54 C. Orendorff and C. J. Murphy, J. Phys. Chem. B, 2006, 110,

3990–3994.55 T. K. Sau and C. J. Murphy, J. Am. Chem. Soc., 2004, 126,

8648–8649.56 S. Mariana and M. Minunni, Anal. Bioanal. Chem., 2014, 406,

2303–2323.57 M. Canovi, J. Lucchetti, M. Stravalaci, F. Re, D. Moscatelli,

P. Bigini, M. Salmona and M. Gobbi, Sensors, 2012, 12,16420–16432.

58 R. Xia, G. Yin, S. Wang, W. Dong, L. You, G. Meng, X. Fang,M. K. Nazeeruddin, Z. Fe and P. J. Dyson, RSC Adv., 2018, 8,17694–17701.

59 X. Zhou, G. Liu, J. Yu and W. Fan, J. Mater. Chem., 2012, 22,21337–21354.

60 R. M. Sarhan, W. Koopman, R. Schutz, T. Schmid, F. Liebig,J. Koetz and M. Bargheer, Sci. Rep., 2019, 9, 3060.

61 R. M. Sarhan, W. Koopman, J. Pudell, F. Stete, M. Rossle,M. Herzog, C. N. Z. Schmitt, F. Liebig, J. Koetz andM. Bargheer, J. Phys. Chem. C, 2019, 123, 9352–9357.

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